Fix a race on AsynchronousPolicyLoader destruction.
Fix a race on AsynchronousPolicyLoader destruction.

Move the delegate_ reset to the file thread in order to avoid racing
against incoming file change events.

BUG=None
TEST=Memory bots still happy.

Review URL: http://codereview.chromium.org/7754031

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@100907 0039d316-1c4b-4281-b951-d872f2087c98

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